This name has never been common in the English-speaking world, though it has been well-used elsewhere in Europe. A famous bearer was the Italian Renaissance master Raffaello Sanzio (1483-1520), usually known simply as Raphael in English.
Myrddin appears in early Welsh poems, as a prophet who lives in the Caledonian Forest after being driven insane witnessing the slaughter of his king Gwenddoleu and his forces at the Battle of Arfderydd. His character seems to be based on the North Brythonic figure Lailoken, and perhaps also the Irish figure Suibhne. Geoffrey of Monmouth adapted him into Merlin in the 12th century.
The name was borne by a semi-legendary 4th-century saint and martyr from Alexandria who was tortured on a spiked wheel. The saint was initially venerated in Syria, and returning crusaders introduced the name to Western Europe. It has been common in England since the 12th century in many different spellings, with Katherine and Catherine becoming standard in the later Middle Ages. To this day both spellings are regularly used in the English-speaking world. In the United States the spelling Katherine has been more popular since 1973.
Famous bearers of the name include Catherine of Siena, a 14th-century mystic, and Catherine de' Medici, a 16th-century French queen. It was also borne by three of Henry VIII's wives, including Katherine of Aragon, and by two empresses of Russia, including Catherine the Great.
Though it has never been popular in the English-speaking world among Christians, it has historically been a common name for Jews, who have used it as an Americanized form of names such as Isaac, Israel and Isaiah.
As an English name it can also derive from the month of August, which was named for the Roman emperor Augustus.